Imaging systems acquire images of a patient, such as images of a suspected tumor, for diagnosis and subsequent treatment or therapy. Commonly used medical imaging systems include fluoroscopy, computerized tomography (CT), magnetic resonance (MRI) or positron emission tomography (PET), for example.
Many imaging systems employ a patient table or couch (the terms being used interchangeably herein throughout the disclosure and claims) upon which a patient is supported throughout the imaging process. In general, the patient lies on the table, which may move along a first axis (generally the azimuth axis). The theoretical isocenter is defined as the symmetry axis of the gantry. The imaging system identifies the spatial coordinates of the suspected tumor, with reference to the isocenter, or for guiding the planning of surgery or other treatments. Accurate measurement of these coordinates is crucial for subsequent treatment of the suspected tumor, because the position of the tumor, as defined by the imaging system, is then used as the target for irradiation, such as by a stereotactic radiotherapy system. A typical stereotactic radiotherapy system uses a linear accelerator (LINAC) gantry, which rotates about the longitudinal axis of the table. It is essential that the isocenter of the LINAC gantry be as close possible to the isocenter of the imaging system.
Multimodality imaging systems employ a plurality of imaging systems, such as CT and PET imaging heads or gantries aligned along a common longitudinal axis. The patient registration should be the same for both imaging systems. Specifically, the isocenter of the first imaging system should be aligned as accurately as possible with the isocenter of the second imaging system. This is not a trivial task because no mechanical assembly is perfect, due, inter alia, to tolerances and the fact that mechanical parts are not infinitely stiff.
Some systems have attempted to solve this problem by means of a fixed alignment between the two imaging systems during the manufacturing process of the multimodality system. Basically this approach places high restrictions on tolerances and mechanical accuracies during production and assembly of the system. This method has the drawback of being quite expensive. Another approach is the use of a fixed calibration during assembly of the system. An accurate jigging fixture is used to align the two imaging systems with each other. This approach is less expensive than the first approach, but still has a disadvantage of being time-consuming. Both methods have a further disadvantage, in that if it is required to service one or more of the imaging systems, the systems must be re-aligned, which can be a cumbersome, time-consuming and tedious task.